The present invention relates to a variable cam engine having cams selectively operated for driving an engine valve depending on the engine operating conditions.
U.S. Pat. No. 5,152,267 (corresponding to Japanese Patent No. 2765218) discloses a variable cam engine adapted for absorbing the torque fluctuation caused upon switching between engine valve driving cams, by retarding an ignition timing. Japanese Patent Application First Publication No. 6-10747 discloses a variable cam engine in which ignition timing (spark) and a fuel injection amount are controlled depending on the change in intake air amount which is caused upon switching between engine valve driving cams.
The above-described related arts, however, do not take into consideration, maximum spark for best torque or fuel economy (MBT) which varies depending on the combustion state before and after switching the cams.
It would therefore be desirable to provide a variable cam engine capable of providing an optimal ignition timing after switching between cams and serving for improving fuel economy and emission control.
In one aspect of the present invention, there is provided a variable cam engine with an intake valve, the engine comprising:
a first cam shaped to allow an increased intake valve lift and an increased intake valve duration;
a second cam shaped to allow a reduced intake valve lift and a reduced intake valve duration;
a cam switching mechanism for switching between the first and second cams depending on an engine speed and an engine load; and
a control unit programmed to substantially equalize amounts of intake air flowing into the engine before and after switching from the first cam to the second cam, and programmed to determine a first ignition timing when the second cam is operated and a second ignition timing when the first cam is operated, and advance the first ignition timing relative to the second ignition timing upon switching from the first cam to the second cam.
In a further aspect of the present invention, there is provided a method for controlling a variable cam engine including a throttle valve, an intake valve, a first cam shaped to allow an increased intake valve lift and an increased intake valve duration, and a second cam shaped to allow a reduced intake valve lift and a reduced intake valve duration, the method comprising:
determining whether the engine is operated in a predetermined range depending on an engine speed and an engine load;
switching from the first cam to the second cam when the engine is operated in the predetermined range;
calculating a target opening degree of the throttle valve at which amounts of intake air flowing into the engine before and after the switching operation are substantially equalized;
controlling the throttle valve to change an opening degree to the target opening degree;
calculating a first ignition timing and a second ignition timing, the first ignition timing being advanced relative to the second ignition timing; and
providing ignition at the first ignition timing when the engine is operated in the predetermined range, and ignition at the second ignition timing when the engine is operated out of the predetermined range.